The repeated use of a mechanical part causes fatigue damage and the part may fracture although it is submitted to a normal stress, the working load not exceeding the load for which the part was intended.
Unfortunately, the fatigue damage of a part is not visible and there does not exist, at the present time, any simple means for determining the fatigue state, which can be the source of serious accidents.
U.S. Pat. No. 4,164,874 describes a fatigue damage indicator comprising two thick members, which are spaced from one another and interconnected by a thin plate provided with a slot, these two members being incorporated in the part or fixed to the latter in such a manner as to be displaced with respect to one another in a plane which is parallel to the plane of the slot.
When the part becomes fatigued, a crack is formed starting from the slot. It is possible to measure the progress of the crack, for example with the aid of a scale engraved on the indicator. But the crack is scarcely visible and the fatigue of the part may well be overlooked as a result.